Rock Island is a city in and the county seat of Rock Island County, Illinois, United States. The original Rock Island, from which the city name is derived, is the largest island on the Mississippi River. It is now called Arsenal Island. The population was 39,018 at the 2010 census. Located on the Mississippi River, it is one of the Quad Cities, along with neighboring Moline, East Moline, and the Iowa cities of Davenport and Bettendorf. The Quad Cities has a population of about 380,000. The city is home to Rock Island Arsenal, the largest government-owned weapons manufacturing arsenal in the US, which employs 6,000 people.